In case you missed the last post, we are building a reflectometer.  The goal is a simple to use, low cost, flexible device for a variety of measurements including tree canopy, soil carbon, and food nutrient density.

If you’re curious about the development process, IRNAS produced a great post walking through the steps from understanding the application to scaled manufacturing.  We are in the rapid prototyping phase, though because we are basing the design on work we already used in the PhotosynQ project, I expect it will be more rapid than usual.  Here’s some updates.

Design Criteria

In order to build a generalized design which can be used in several applications, we tried to build a device which could measure several types of objects effectively.  In order to measure reflectance effectively, the background behind the object of interest must be consistent – variation n the background will produce noise in the signal.  The mechanical design addressed this (see image below).  Here’s the list of materials this device is designed to measure.

Drop of liquid – drop from crushed leaf, drop from crushed food. refraction, brix.
Small cuvette of liquid – juice, chemical mixture. Reflectance at all wavelengths, density, colorimetry
Bulk solid – soil.  Soil carbon.
2D object – leaf, paper. Chlorophyll content
3D object – fruit, vegetable. Reflectance at all wavelengths, correlated to nutritional content (This use cannot utilize a consistent backdrop due to size and cannot require a guaranteed distance to sample)

In many projects, signal quality can be defined from the start.  However, here it’s hard to say if we need less than .5% noise or less than .05% noise without simpling testing on actual samples.  So, we will have a reference set of objects (colored liquids, bulk solids, colored paper or different thickness, etc.) to confirm that we are hitting the quality needed as we iterate on the design.  We are building that reference set now, so when we have our first prototype we will be ready to put it through it’s paces!

The next step was to produce a schematic.  That’s just a fancy word for a wiring diagram – which parts connect to which other parts.  We use a program called KiCAD, which is free and open source and amazing.  Here’s a picture of our schematic for the reflectometer.

Once that’s complete, then you actually have to wire it up.  Here’s what the actual circuit board looks like.

You then send this layout to a company which manufactures the board.  Next, someone has to put all the parts of the board – in our case we have about 200 components that need to be soldered onto the board.  Until recently, that’s been the slowest part of rapid prototyping, and causes lots of delays because hand soldering producers errors and takes time and having an outside contractor do it took too much back and forth and time.  But now companies provide fast turnaround (2 weeks!) on fully populated boards, and have huge stock inventories so they don’t often have to order your parts at all – they are all already in house.  That means you can prototype faster and cheaper than ever before.

We are less than a week away from shipping this design out to be routed and populated.  A few weeks after that we should have boards in hand, ready to test against our standards.  If we find we exceed our standards, then we can begin to identify places for cost savings, hopefully getting the board into the $50 in parts range.

If you want to download the schematic, you can find it on our gitlab page – https://gitlab.com/our-sci/reflectance-spec-PCB.  More updates in a few weeks.